1. Field of the Invention
The present invention generally relates to printing heads, and more particularly to an ink jet type printing head which is applied to image recording apparatuses such as printers and facsimile machines.
2. Description of the Related Art
The conventional ink jet type printing head is provided with a nozzle, a pressure chamber, an ink supply passage and an ink tank, and ink particles are ejected from the nozzle by generating pressure in the pressure chamber so that characters or images are recorded on a recording paper. As means of applying the pressure to the pressure chamber, the generally known system adheres a piezoelectric element on the outer wall of the pressure chamber and uses the displacement of the piezoelectric element which is generated by applying a pulse voltage to the piezoelectric element. FIG. 1 is a diagram for explaining the general structure of the conventional printing head using this system. In FIG. 1, an ink 2 fills a pressure chamber 1, and a piezoelectric element 3 is adhered on an outer wall 1a of the pressure chamber 1. One end of the pressure chamber 1 communicates with a nozzle 4, and the other end is connected to an ink tank which is not shown.
FIGS. 2A and 2B are diagrams for explaining the function of the printing head shown in FIG. 1. FIG. 2A shows a state where the voltage is applied to the piezoelectric element 3 and the ink is about to be ejected from the nozzle 4, and FIG. 2B shows a state where ink particles (i.e., droplets) 5 are ejected.
On the other hand, as shown in FIGS. 3A through 3E, there is a system of using a thermal resistor element 7 which is positioned in the vicinity of a nozzle 6 as the pressure generating means. In the printing head employing this system, a pulse voltage is applied to the thermal resistor element 7, and bubbles 8 are generated within the ink by the heat which is generated, so as to eject ink particles 9 from the nozzle by this pressure. FIG. 3A shows an initial state of the bubble generation, FIG. 3B shows a state where bubbles are generated to a certain extent, FIG. 3C shows a state where the bubble has grown large and the ejection of the ink is about to start, FIG. 3D shows a state where the ink ejection has progressed further, and FIG. 3E shows a state where the ink particles 9 have been ejection.
The conventional ink jet type printing heads described above are suited for use in offices because they generate far less noise compared to the wire dot printing heads, i.e., dot matrix printers, which print by pressing wires against a platen via an ink ribbon and the paper.
However, the conventional ink jet type printing heads suffer from the following disadvantages.
That is, in the case shown in FIGS. 1, 2A and 2B, the entire printing head must be replaced because dust particles and the like adhere to the nozzle of the printing head, air bubbles enter from the nozzle or the nozzle becomes blocked by dried ink.
In addition, even in the case shown in FIGS. 3A through 3E, similar problems existed because the head generating part, i.e., pressure generating means, is integrally formed on the nozzle and the pressure chamber.
Recently, a disposable head integrally having a printing head and an ink head in the form of a head cartridge has been developed, and the entire head cartridge is replaced when all of the ink within the tank is consumed. However, according to such a printing head, the pressure generating means is disposed of at the same time, and there are problems in that the cost of the head is high and the running (i.e., operating) cost is high.